SECTION II PROCESSES OF WOOD DECAY AND DETERIORATION

SECTION II PROCESSES OF WOOD DECAY AND DETERIORATION

PROCESSES OF WOOD DECAY AND DETERIORATION INTRODUCTION Environmental safety has emerged as the number one issue of the 1990's. Unfortunately, the major wood preservatives- creosote, penta-chlorophenol, and inorganic arsenicals - are broad-spectrum poisons that pose a serious threat to the environment. These chemicals have already been banned or restricted in some countries. At present, no harmless substitutes for toxic chemicals are available. Thus, we urgently need new, novel approaches for preventing wood deterioration. Consumer concern about environmental damage may lead to untreated wood being used in applications where it is subjected to biodeterioration. The unsophisticated approach of using metabolic poisons can only be phased out if new and more selective alternatives are developed. This can best be accomplished if we identify and take advantage of unique features of degrading organisms or their biochemical pathways. This necessitates a thorough knowledge and understanding of the fundamentals of wood decay, including mechanisms and methods of deterioration, as well as conditions conducive to fungal proliferation and growth. If researchers understand the biochemical agents in fungal attack that are responsible for degradation, they will be able to "sabotage" the ability of these agents to decay wood. Focusing research on physiological differences between the wood-attacking fungi and nontarget organisms contributes to environmental safety; we can specifically interfere with fungal activity. The research reported in this chapter discusses several different aspects of fungal deterioration of wood, which should aid in the development of safer wood preservation methods. Brown-rot fungi cause the most destructive for of wood decay, but the mechanism by which brown-rot fungi degrade wood is not well understood. 227

These fungi are unique in that they are the only known microbes that can degrade cellulose in wood without first removing the lignin. Furthermore, they degrade cellulose in an unusual manner, causing rapid decrease in depolymerization at low weight loss. A review paper addressed chemical changes in wood components and cotton cellulose as a result of brown-rot degradation. the literature indicates that chemical changes in brown-rotted lignin are oxidative in nature and result primarily in hydroxylation and demethylation of the lignin polymer. Results from studies of the chemical changes in cellulose as a result of brown-rot attack do not unequivocally demonstrate oxidative degradation. The Fenton's reagent (H 2 0 2 /Fe + +) has been hypothesized as a model by which brown-rot fungi oxidatively depolymerize cellulose. However, a review of the effects of Fenton's reagent on wood components and model compounds suggests that the effects of brown-rot decay are not duplicated by this reagent in many respects and, therefore, it does not represent and adequate model for brown-rot decay. The extracellular enzymes produced by wood-decay fungi are important in digestion of wood cell wall constituents. Knowledge of these enzymes is beneficial for (a) inactivating them to prevent decay, (b) early detection of decay by producing antibodies to degradative enzymes, and (c) industrial and agricultural bioconversion processes. However, extracellular wood-decay enzymes are difficult to study because they are produced in such small amounts. One paper addressed an attempt to overproduce and enhance the secretion of lignocellulolytic enzymes from the white-rot fungus Trametes versicolor. In the development of the immunodiagnostic methodologies and inhibitor studies for decay prevention, using enzymes that have the same properties as those produced in wood during decomposition is particularly important. To determine if the culture medium affects enzyme characteristics, another paper compared the antigenic properties of the extracellular enzymes from the brown-rot fungus Postia placenta grown in liquid culture, malt agar, or wood. The authors concluded that the antigenic characteristics of extracellular polysaccharidedegrading enzymes of f.,_ placenta and substrate dependent and that degradative enzymes produced in vitro differ from those produced in wood. Earlier studies of decay by white-rot fungi suggested that the faster decay of hardwoods than that of softwoods may be partially explained by the effect of the different amounts and different types of lignin in these woods. The lignin of 228

most softwoods is composed almost entirely of quaiacylpropyl (G) units. Lignin in hardwoods, in addition to G units, often has numerous syringylpropyl (S) units. A study addresses the influence of lignin type on decay by the white-rot fungus Trametes versicolor by using a number of woods with a wide range of S:G ratios and different lignin type distributions. The results of the study are consistent with the general premise that lignin concentration and lignin type affect the decay resistance of wood. However, not enough experimental evidence was obtained to prove that lignin character is the limiting determinant of decay. In fact, the syringyl content of the lignin had no general correlation with the rate of decay. Using biological controls to protect wood against fungal attack is an environmentally safer alternative to chemical biocides. Biological control of wood decay exploits ecological relationships among organisms by using natural antagonists against wood-attacking fungi. these papers addressed the use of biological controls to prevent decay, mold and stain attack in wood. One study determined the ability of a commercial biological formulation containing Tricoderma propagules to colonize and prevent decay in Douglas-fir and Southern Pine timbers exposed above ground. Tricoderma colonized Southern Pine timbers but did not prevent decay. Douglas-fir timbers were poorly colonized by Tricoderma. The author concluded that the use of biological control agents to control wood-attacking fungi should emphasize short-term protection, such as protection of lumber and logs during drying. For a biological control organism to provide long-term protection against decay in wood products would be very difficult. The biological control organism would have to spread throughout the wood and deposit a residual fungistatic material that remains after the organisms death or alter the substance to make it nutritionally undesirable to the decay fungus. A second biological control paper investigated the suitability of using the metabolites from the bacterium Streptomyces rimosus, for controlling mold and stain fungi on green wood during drying. The results of this study showed that S1.. rimosus produced very potent antifungal metabolites that effectively prevented mold and stain attack on wood in laboratory tests and on green pine log sections in simulated field trials..s.t._ rimosus has been reported to produce the antifungal antibiotic, rimocidin, but the authors have not yet identified the antifungal metabolites responsible for mold and stain prevention. If antifungal 229

metabolites from St. rimosus are to be considered for commercialization, identifying them as soon as possible is essential. The third paper on biological control reported the effects of the bacterial metabolite isatin (2, 3-indolinedione) on white- and brown-rot decay in vitro and discusses the relevance of the effects of fungal metabolism. I satin concentrations above 5mM inhibited the growth of 12 wood-decay fungi. The effect of lower isatin concentrations on wood-decay and parasitic fungi implicated that isatin intervened in metabolic pathways responsible for fungal growth and development. The fact that isatin had a physiological effect on wood decay fungi and that toxic levels were identified for application is relevant. lsatin should be included in the search for microbial allelochemicals. Compounds derived from natural products may be safer for the environment that synthetic chemical compounds and would be more acceptable to consumers. Another paper determined the fungitoxicity of extratives from cones from four different Pinus species against mold, sapstain, and wood-decay fungi. The C0 2 fraction of diethyl ether extracts of four species of pine cones inhibited the growth of representative mold, sapstain, and wood-decay fungi. However, none of the C0 2 fractions inhibited fungal growth entirely. A large portion of the fungitoxicity was probable due to the presence of abietane resin acids. Abietane resin acids have an isopropyl group attached to C-13 that may have been responsible for the fungitoxicity of these compounds. Wood protection in the future will depend less on the use of broadspectrum metabolic poisons. With more sophisticated tools and techniques to aid researchers in studying problems of wood degradation and protection, our Understanding of decay processes should advance rapidly in the coming years. Biochemical and physiological studies of wood decay will reveal new and exciting information with essential biochemical processes and develop highly active and sharply targeted preservatives with enhanced preservative performance and safety. Terry L. Highley USDA Forest Service Forest Products Laboratory Madison, WI 53705-2398, USA 230